Just to be clear, it doesn't need an add-on such as Google SSL or HTTPS Everywhere to still give you those types of results?

Yup. By default ddg is redirecting you to https page, if there is valid SSL cert.

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Can my search return HTTPS only for DuckDuckGo searches?

Nope. There is no any option to sort results only by https and eliminate non-https result, because this would drastically reduce result page. (lack of data)
However, this is interesting idea and hopefully it will be implemented in future.